Unrepeated transmission of optical data over long fiber spans is of interest in a number of applications, such as fiber cable links between cities, or links undersea. In both instances, it would be very costly to extract the cable system for repair. It is therefore of great interest to develop optical transmission systems which have no active parts, such as booster amplifiers, which consume power and need periodic repair and replacement. Damen et al., U.S. Pat. No. 5,737,460, presents an optical transmission system which uses solitons, to carry data. Solitons can propagate extraordinarily long distances without distortion, and are thus excellent candidates for long distance repeaterless systems. Unfortunately, the Damen et al. invention has a limited upper bandwidth, which in turn limits the speed with which data encoded on solitons can be transmitted. One way to increase bandwidth in any digital communication system is to wavelength division multiplex the bits, that is generate bits having spectra with differing center frequencies, launch all the bits on one line, and then use filter banks to separate the bits by frequency and separately detect them. Thus if one uses N multiplex channels with a corresponding N detectors for each channel operating at maximum detection rate, one has increased system data rate by a factor of N. Present commercial data systems do this using Non-Return-to-Zero encoding. Unfortunately, Non-Return-to-Zero data bits of different frequencies in this scheme are given to distortion and cross-talk due to the fiber Kerr nonlinearity, and hence are inherently given to signal degradation. Soliton encoded signals of different frequencies have a similar difficulty in that they will distort one another if they collide asymmetrically, that is if they do not pass completely through one another. To avoid inter-soliton distortion, one must launch the solitons far apart to ensure that they have no appreciable temporal overlap at time of generation, so that subsequent collisions will be symmetrical. Unfortunately, this implies a low transmission rate.
Accordingly, an object of the invention is to permit optical communication over long distances by a system that is passive, requiring active components only at system termini.
Another object is to do this using optical solitons.
Another object is to increase the upper bandwidth of such a system, and hence its upper data rate.
Another object is to do so by wavelength division multiplexing.
Another object is do so with a system that is resistant to bit distortion and inter-bit cross-talk.
Another object is to do the foregoing in a manner to ensure that the solitons collide only symmetrically.
In accordance with these and other objects made apparent hereinafter, the invention concerns an optical communications system having an adiabatic link, a pulse generator which launches temporally interleaved solitons into the adiabatic link, and a receiver with a wavelength division de-multiplexer. Each pulse has a unique center frequency but bandwidths which substantially overlap. The adiabatic link causes the solitons"" bandwidths to narrow such that at the receiver the bandwidths are substantially distinct, permitting detection of each soliton by the wavelength division de-multiplexer.
Because the wavelength division multiplexed solitons are interleaved at launch, i.e. launched separately rather than in a virtual state of asymmetric collision, the solitons will not mutually distort each other as they initially disperse. Wavelength division multiplexing the solitons secures an increase in system data rate proportional to the number of multiplex channels used; interleaving the solitons, rather than generating one soliton per pulse from the generator, further increases the data rate without the cross-channel distortion in other communication systems, such as that based on Non-Return-to-Zero encoding.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: